Sequential Prophet X: read and write the sample instruments (GRP, ZIP)

[douglas-carmichael]

Adds the sample instruments of the Sequential Prophet X and Prophet XL as a source and a destination. An instrument is a folder with its WAV files and a tab-separated group file (.grp); the device imports it as px/<user bank>/<NN Category>/<name>.zip from a USB drive, which is the layout of the official 8Dio mapping utility and of the free PXToolkit.

Format. Sequential does not document it. The reader of the device was read out of the OS 2.2.2 update image, an unstripped x86-64 QNX executable (GroupFileParse, CWavReader::Open, CSampleGroup::FindHandler, LibraryCopy_ImportFolder): the header line names the columns in any order, the loop end is inclusive, a round robin set is the samples which share key and velocity range and carry a number, Pitch in Hertz is the frequency at which the file sounds and the playback rate is the ratio of the frequency of the key to it, and the sample rate of a WAV file is never read - a file at another rate than the 48 kHz of the engine plays transposed. Volume.txt replaces the built-in velocity table of the device. documentation/design/PROPHET_X_GRP_FORMAT.md records all of it, including what the format cannot carry: the modulation belongs to the programs of the device, and a play range does not exist.

Reading takes the instrument folders and the import archives: key and velocity ranges, loops, round robin sets as groups, root key and tuning from the frequency, key tracking, the category from the folder, the name from the Instrument Name column, and Volume.txt fitted to the amplitude velocity modulator (depth and curve). The empty samples with which PXToolkit keeps unmapped keys silent are skipped.

Writing creates the import archive, or a plain folder, with 16 bit / 48 kHz WAV files stripped to their format and data chunk, the play range of a zone cut into its audio, round robin numbers, an empty sample for the keys which no zone covers - the device plays the nearest zone on them otherwise - and Volume.txt when the velocity response differs from the default. The category is the closest of the 17 categories of the device to the category of the source, or the selected one; the user bank is an option as well. A UUID derived from bank, category and name gives a source which is converted again the same identity.

Checked:

• A synthetic SFZ (two key zones, two velocity layers, two round robins, 24 bit 44.1 kHz and 16 bit 48 kHz samples, loops, a tune of -20 cents, an offset, 80 % velocity with a cubic curve) through the writer and back through the reader: every value returns, the loops scaled to 48 kHz, the offset cut in.
• The six archives of the free Goldbaby GBPX Free1 pack, made with PXToolkit for the device: all read (categories from their folders, four round robin groups on the harp, whole-file loops on the noise instruments), and written again after a pass through SFZ every key and velocity range, round robin number and pitch matches the original.
• The writer of PXToolkit (ProphetWriter.js) is the reference for the conventions above; the written archives are not verified on a device, since none was available.